With the high advance of electronic industries, the integration degrees of semiconductor memory devices are increasingly required. The integration degree is a significant factor in the determination of product prices. For example, as the integration degree increases, the price of the semiconductor memory devices may be reduced. For this reason, higher integration degree may be especially required for the semiconductor devices. Generally, since the integration degree of the semiconductor devices may be determined by a two dimensional area of a unit memory cell, the integration degree may be considerably affected by the level of technique for forming fine patterns. However, there may be a limit to fining the patterns due to high expensive equipments and/or difficulties in semiconductor fabricating processes.
Recently, the semiconductor memory devices with a three-dimensional structure are suggested in order to overcome these limitations. These new structures, however, may bring about various problems such as deterioration in reliability and/or operational speed of products. Therefore, much research has actively been undertaken in order to overcome these problems.